How to Use Marine Radar for Collision Avoidance

A Practical Guide to Marine Radar Operation for Powerboat and Motor Cruiser Skippers

Marine radar is one of the most powerful navigation tools available to small craft operators, yet many boat owners either do not use it effectively or avoid using it altogether because it seems complicated. This guide introduces the fundamentals of radar operation, focusing on the collision avoidance applications that make radar so valuable for safe navigation.

While this article provides an overview, the best way to develop genuine radar competence is through formal training with practical exercises. The RYA RADAR Course offers exactly this, combining theory with hands-on simulator practice.

How Radar Works

Radar stands for Radio Detection and Ranging. Your radar transmits pulses of radio energy from a rotating antenna (the scanner). When these pulses strike a solid object, some energy reflects back to the antenna. The radar measures the time between transmission and reception to calculate distance, while the antenna’s rotational position determines bearing.

The result is displayed on your screen as a plan view of the area around your vessel. Your boat is at the centre, and solid objects appear as bright marks (targets or echoes) at their correct bearing and distance.

Different objects return different strength echoes. Large metal vessels return strong echoes visible at long range. Small fibreglass boats return weaker echoes that may only be visible at closer range. The coast, with its mix of cliffs, buildings, and vegetation, creates a complex pattern of returns.

Understanding this basic principle helps you interpret what the radar is showing you. A target is not a photograph of another vessel; it is the radar’s interpretation of reflected radio energy.

Setting Up Your Display

Before you can use radar effectively, you need to set it up correctly for the conditions. Modern radar systems have numerous controls, but several are fundamental.

Range: This sets how far the radar looks and how large an area is displayed on screen. For collision avoidance in open water, use longer ranges (6-12 miles) to detect vessels early. In confined waters or busy areas, shorter ranges (1-3 miles) provide more detail. Many operators check long range periodically for early warning, then use shorter range for detailed navigation.

Gain: This controls the receiver sensitivity. Too little gain and you miss weak targets. Too much gain and the display fills with noise (speckle). Adjust gain until you see a slight amount of speckle across the display, which indicates the receiver is sensitive enough to detect weak echoes.

Sea clutter: Waves reflect radar energy, creating a pattern of returns around your vessel called sea clutter. The sea clutter control reduces these returns, but be careful: turning it up too high can eliminate genuine targets hidden in the clutter. Adjust to minimise clutter while preserving targets.

Rain clutter: Heavy rain also reflects radar energy, appearing as areas of interference on the display. The rain clutter control (sometimes called FTC for Fast Time Constant) helps reduce this, but again, excessive adjustment can eliminate targets.

Display Modes

Marine radar can display information in several orientations, each with advantages and limitations.

Head-up: The direction your bow is pointing is always at the top of the display. This is intuitive because targets on your right appear on the right of the screen. However, when you alter course, the entire picture rotates, which can be disorienting and makes it harder to track targets systematically.

North-up: North is always at the top of the display. The picture remains stable when you alter course, making systematic target tracking easier. However, targets are not necessarily in the direction they appear on screen; you need to mentally correlate the radar picture with what you see outside.

Course-up: Your intended course is at the top of the display. This provides a stable picture like north-up, but aligned with your direction of travel.

For collision avoidance, north-up or course-up is generally preferred because the stable display allows reliable target tracking. Head-up is useful for immediate orientation but makes systematic observation more difficult.

Tracking Targets

Collision avoidance requires systematic observation of targets, not just noting their position at a single moment. You need to track bearing and distance over time to determine whether collision risk exists.

A simple method is to note a target’s bearing and distance at regular intervals, perhaps every three minutes. Plot these observations on paper or observe the pattern. If bearing remains constant while distance decreases, you are on a collision course, regardless of what the other vessel appears to be doing.

Modern radar systems with ARPA (Automatic Radar Plotting Aid) or MARPA (Mini Automatic Radar Plotting Aid) can automate this tracking. Once you acquire a target, the system calculates and displays CPA (Closest Point of Approach) and TCPA (Time to CPA). This tells you how close the target will pass and when.

However, automatic tracking is not infallible. Targets can be lost if echoes fade. The system may confuse multiple targets in close proximity. You should understand manual tracking methods even if your radar has automatic features.

Determining Collision Risk

A target presents collision risk if its CPA is too close for comfort. What counts as “too close” depends on the circumstances, including visibility, sea state, vessel types, and your own comfort level. In restricted visibility, many operators consider anything under one mile as requiring attention.

The classic collision scenario is the target with constant bearing and decreasing range. On radar, this appears as a target that stays at the same clock position (relative to your bow) while the range rings show it getting closer.

Watch for targets whose bearing changes very slowly. A target moving slowly across your screen may still have a dangerously close CPA if it is large or travelling fast.

Taking Avoiding Action

Once you identify a collision risk, you need to take avoiding action early and decisively. Small course alterations are hard for other vessels to detect; make substantial changes so your altered heading is clear.

When altering course, monitor the effect on the target. If you turn to starboard and the target draws aft (moves towards your stern), your avoidance is working. If it continues to track forward, you may need more substantial action.

In restricted visibility, Rule 19 of the collision regulations provides specific guidance. Avoid turning to port for vessels forward of your beam (except when overtaking). Avoid turning towards vessels abeam or abaft your beam. These rules reduce the risk of both vessels turning towards each other.

Using Radar for Navigation

Beyond collision avoidance, radar is valuable for navigation and pilotage. You can take range and bearing to identified features (headlands, buoys, distinctive structures) to fix your position, particularly useful when GPS accuracy is questionable or when you want to verify your electronic position.

Parallel indexing is a powerful pilotage technique where you create a reference line on your display and monitor your progress along a planned track. If the coastline or a specific feature maintains its position relative to your index line, you know you are on track.

Limitations to Understand

Radar has limitations that can mislead the untrained operator.

Shadow sectors: Your own vessel’s structure can block radar transmissions in certain directions, creating blind areas where targets will not be detected.

Target discrimination: Two targets close together may appear as one. A small vessel close to a large one may be hidden.

False echoes: Multiple reflections can create targets that do not exist. Sidelobe returns can show targets at incorrect bearings.

Sea and rain clutter: Rough seas and heavy rain can mask genuine targets if you are not careful with control settings.

Small targets: Fibreglass boats, small dinghies, and even people in the water may not return detectable echoes, especially in rough conditions.

Understanding these limitations prevents over-reliance on radar. It is a powerful tool, but not an all-seeing eye.

Building Your Skills

Effective radar use requires practice, not just reading. The RYA RADAR Course at Ocean Sports Tuition provides exactly this. Over one day, you work through progressive simulator exercises that build your skills from basic setup through to complex collision avoidance scenarios.

Based at Saxon Wharf Marina in Southampton, we offer radar training that prepares you for real conditions in the Solent and beyond. To book your course, call 02381 242159.

Consider combining radar training with RYA Yachtmaster Theory for comprehensive navigation knowledge, or the RYA Advanced Powerboat Course for practical experience including night navigation where radar skills prove their worth.

For more information about the RYA RADAR Course syllabus, see the official RYA course information.